I am stumped. Last year I replaced the instrument panel voltage regular with a solid state one I built. I installed it after disabling the built-in one in the fuel gauge. All was well. Then after a year the fuel gauge started reading low so I installed another one (after disabling the built-in voltage regulator in it). Still read low. So I replaced the sending unit. Both gauges still read low. So I checked the resistance of the new sending unit with a full tank - 9 ohms. Then I made sure this was the resistance at the fuel gauge and it was. Then I checked the output of my hand built voltage regulator - 5 volts exactly. Both gauges read low. So I got another gauge and it reads low. Can all three gauges be broken? Temp gauge works well. Any help would be appreciated.

The factory's voltage limiter usually put-out 5-7 volts with a pulsing action.
The last solid state limiter I made had adjustable voltage and I had to run slightly more then 5 volts to get the gauges to read correctly.
DD

Now that's good information that I didn't know. I'm still looking at instrumentation options and may well upgrade to modern gauges before it's all said and done but I'll file this tidbit away for possible future reference.

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David Kight
'62 Valiant Signet, White
'98 Dodge Dakota
'06 Jeep Liberty

Growing older is unavoidable but growing up is strictly optional.

The LM7805 5V regulator is very reliable and I've never seen one go partially bad. Since the regulator is putting out 5V now and did before, I doubt that it's the problem. I would carefully check the grounding of the dash.

You can do shift the output of a LM7805 up or down by using a diode.

Thanks for all the information. The gauges are properly grounded and so is the sending unit. I guess I'll try upping the voltage from the regulator.

Just as a follow up to this knarly problem. I built a variable voltage output regulator and installed it. Running at 5.8 volts the fuel gauge works perfectly. However, at that voltage the temp gauge read way high. At any lower voltage, like 5.0 volts, the fuel gauge read low. After pulling my hair out for a while I decided to use 2 regulators, one at 5.8 for the fuel gauge and the one I had previously built at 5.0 for the temp gauge. I hated to do that, but it seemed the best solution since I had the two gauges in hand. As long as both regulators continue to work (and they should being solid state) I'm ok.

A high resistance anywhere in the circuit can cause the gas gauge to read low - I'd check the tank grounding 1st, then look for corroded connector, then a damaged wire in the harness. Turning the voltage higher will compensate until the connection gets worse.

It can also be the gauge itself.
Sounds like you have a "fix" but keep your eye out for some extra gauges, they may come-in handy later.
DD

Just for your information:

I checked the grounding of the sending unit at the tank.

I tried 3 different gauges, all read the same.

I checked and rechecked the resistance supplied to the gauge by the sending unit.

This solution was my last resort.

Jack: Is it possible your fuel sender float isn't floating as much as it used to? Or the arm the float is attached to has gotten bent 'down' some when being inserted into the tank? Good luck to you.

Actually, what I neglected to say was that I also installed two different sending units. The one that is in there is brand new. It shows 3.5 ohms when the tank is full. With 3.5 ohms at the gauge, proper grounding of the gauge, and 5.0 volts into the gauge, the gauge reads low, just over 3/4 full. I puzzled over this for a long time and put tons of time into it. I thought with an adjustable voltage input that this would solve the problem, but the damn temp gauge didn't work at anything higher than 5.0 volts. Since I disabled the stock voltage regulators on all 3 gauges that I have, I can't test the stock setup. But, for now, I have the problem solved using 2 voltage regulators. I think this is a case of just getting it to work, rather than getting a real understanding of the problem. Thanks for all the advice.